MOLLUSCA 



361 



are the rudiments of the funnel. The retractor muscles of the 

 funnel appear as two ridges which stretch from the middle of the 

 funnel, where anterior and posterior funnel folds have united, 

 upwards to the edge of the mantle. The mantle edge extends over 

 the rudiments of the gills, and the mantle cavity deepens ; whilst 

 the shell-sac closes, and the central region of the mantle becomes 

 arched up so as to constitute a visceral hump. The rudiments of 

 the arms lengthen ; those of the tentacular arms exceed the others 



FIG. 293. Sagittal sections of two embryos of Loliyo ntlgaris to illustrate the formation 



of internal organs. (After Korschelt. ) 



A, Section of younger embryo with incomplete alimentary canal. 13, Section of older embryo in 

 which stomodaeum and niid-^ut have joined, a, anus ; <\g, cerebral ganglion ; e.y.a, external yolk-sac ; 

 /, funnel ; //;,, tin rudiment ; //, heiirt ; int., intestine ; i.x, ink sac ; t.J/.s, internal yolk-sac ; m.c, mantle- 

 cavity ; HI./, mantle-fold ; u, mouth ; ut, otocyst ; ji.ij, pedal ^an^lion ; j, K r, pericardium ; r.s, radulasac ; 

 sal, salivary gland ; s.s, shell-sac ; stout, stomodaeum ; t:g, visceral ganglion ; y.-in, yolk -membrane. 



in length. The rudiments of the ganglia become constricted off 

 from the thickenings of the ectoderm ; that part of the thickenings 

 giving rise to the cerebral ganglia which lie beneath the eyes, becomes 

 infolded and gives rise to the so-called white bodies (Fig. 292). 



The internal changes which occur are best made out by combining 

 the views obtained by sagittal sections with those obtained by 

 horizontal sections through the embryo. The most marked features 

 of the stage which we are discussing are the appearance of the 

 coelomic cavities and of the blood spaces, both of which arise as 



